1. Technical Field of the Invention
The present invention relates to electric rotary machines and, more particularly, to a vehicular tandem type rotary electric machine having a single rotary shaft and plural stator rotor pairs.
2. Description of the Related Art
Attempts have heretofore been made to provide rotary electric machines (hereinafter referred to as vehicular tandem type rotary electric machines) each of which includes two Lundell type rotors coupled in tandem for a doubled power output as disclosed in Japanese Patent Laid-Open Publication Nos. 1-157251, 5-137295, 5-308751, 5-500300, 6-22518, 11-98789 and 2005-117843. With these tandem type rotary electric machines, the rotary electric machines can be manufactured in a compact structure arranged to provide two output voltages at different levels independently controllable from each other. Stated another way, in contrast to a structure in which two rotary electric machines are individually installed, the tandem type rotary electric machine can remarkably reduce production cost and installation space. The possibility of individually controlling the two output voltages is particularly useful for a two-voltage type rotary electric machine that is operative to separately provide a high output voltage of, for instance, 42V in addition to a low output voltage of 12V.
Further, another attempt has heretofore been made by the same applicant as that of the present application to provide tandem type rotary electric machines each arranged to include a segments-sequentially-joined stator coil composed of U-shaped segment conductors inserted to a slot formed in a stator core as disclosed in Japanese Patent Laid-Open Publication Nos. 2004-048939, 2004-048941, 2004-064914, 2004-048967, 2004-032987, 2004-032882, 2004-032884 and 2004-032890.
With the tandem type rotary electric machines incorporating a plurality of Lundell type rotor cores disposed in an axial direction, an issue arises with a remarkable increase in an axial length in comparison to that of a commonly used vehicular alternator employing a Lundell type rotor core.
Such an increase in the axial length of the rotary electric machine leads to an increase in the occurrence of deflection of a rotary shaft in a radial direction thereof. This results in a need to increase an electromagnetic gap between the stator core and the rotor core in a radial direction thereof. However, Such an increase in the electromagnetic gap in the radial direction reflects an increase in magnetic flux reluctance of a magnetic flux circuit with the resultant drop in performance of the rotary electric machine. Further, the increase in the axial length of the rotary electric machine also causes an increase in a rotating mass and a need arises for the rotary shaft to be largely sized in diameter and for bearings and housing to be designed in increased strength with a view to minimizing vibrations of relevant component parts. This results in the occurrence of an issue with an increase in an overall weight of the rotary electric machine.
Consideration has been taken that the increased axial length of the tandem type rotary electric machine provides an insoluble issue inevitably caused in the tandem type rotary electric machine of the related art needed to array the two stator-rotor pairs in the axial direction. Therefore, such a factor reflects a major cause for the tandem type rotary electric machine not to be widely spread in the related art regardless of various advantages.